Korean nannastacids have recorded only eight species belonging to genera Campylaspis and Cumella. In this paper, Nannastacus nyctagineus Gamȏ, 1962, is redescribed and illustrated as a new species of Korean fauna in this study for the first time. Korean female specimens of Nannastacus nyctagineus correspond well with the original description by Gamȏ, 1962 based on only female specimens. Gamȏ has described that new species, Nannastacus pruinosus could be distinguished from N. nyctagineus based on only male specimens. The Korean male specimens of N. nyctagineus undoubtedly match well with the description of N. pruinosus with the following common features: the surface of the carapace is covered with numerous granules; pereonites and pleonites have similar patterns of the teeth on dorsal surface; the proportion and armature of the dactylus of pereopod 2 is similar; the number and pattern arrangement of the setae on the surface in antenna 1, pereopod 1, and uropod are the all most same. Nannastacus pruinosus Gamȏ, 1962 is proposed as a synonym of N. nyctagineus described in this study. This represents the first record on the genus Nannastacus from Korea.

Family Nannastacidae Bate, 1866 has 25 genera with approximately 430 species worldwide (Gerken, 2012). Among the 25 genera, Campylaspis Sars, 1865, Cumella Sars, 1865, and Nannastacus Bate, 1865 are the main ones in this family. However, Korean nannastacids have only recorded eight species belonging to genera Campylaspis and Cumella(Lee and Lee, 1999, 2001, 2012, Lee et al., 2012). In this paper, Nannastacus nyctagineus Gamȏ, 1962, is described and illustrated as new to Korean fauna. The specimens were collected mainly using a light-trap (Holmes and O’Connor, 1988; Kim, 1992) in shallow Korean waters, from 1993 to 2004. Body length was measured from the anterior tip of the carapace to the posterior end of the pleonite 6. The lengths of the appendages were measured along the mid-line of each appendage and excluded the inflated outer angle. The materials are deposited at the Department of Life Sciences, Dankook University, Cheonan, Korea.

Pereopod 1 (Fig. 1C): basis 0.45 times as long as remaining articles combined, with 1 row of serrations and 1 simple proximal seta medially. Ischium with 2 simple setae. Merus with 3 simple setae. Dactylus 0.70 times as long as propodus, with 2 simple and 1 long stout setae terminally.

Uropod (Fig. 1F): peduncle short, subequal in length to half of pleonite 6, inner margin with numerous hairs. Endopod not articulated, 2.40 times as long as peduncle, inner margin serrated, with 1 short simple, 2 complex pedunculate, and 1 small stout setae; terminal margin with 1 small simple and 1 long stout setae. Exopod not articulated, very short, subequal to 1/9 length of endopod, with 1 short and 1 long stout setae (reaching to about 5/6 point of endopod) terminally.

Antenna 1 (Fig. 2C): peduncle composed of 3 articles; first article slightly shorter than remaining articles combined, with numerous hairs, and with 11 simple distal setae in outer margin; second article 0.45 times as long as first one, with 2 simple setae on outer corner; inner corner very inflated, longer than 1/2 length of third article, with 3 complex pedunculated setae distally; third article subequal in length to second one, with 3 complex pedunculate setae on distal margin. Main flagellum composed of 2 articles, slightly longer than last article of peduncle; second article with 1 simple seta and 2 aesthetascs terminally. Accessory flagellum not articulated, with 2 simple and 1 complex pedunculate setae.

Pereopod 3 (Fig. 3B): basis 0.90 times as long as remaining articles combined.

Pereopod 4 (Fig. 3C): basis 0.75 times as long as remaining articles combined.

Pereopod 5 (Fig. 3D): basis 0.45 times as long as remaining articles combined.

Uropod (Fig. 3E): peduncle 0.86 times as long as pleonite 6, with numerous hairs on inner margin. Endopod not articulated, 2.50 times as long as peduncle; inner margin serrated, with 5-6 small stout setae; terminal margin with 1 small simple and 1 long stout setae. Exopod not articulated, very short, subequal to 1/13 length of endopod, with 1 short setae and 1 long stout seta (reaching to about 3/5 point of endopod) terminally.

Remarks. Korean female specimens of Nannastacus nyctagineus correspond well with the original description by Gamȏ, 1962 based on only female specimens collected from Japanese water (the Tanabe Bay, Kii peninsula). Gamȏ has described that new species, Nannastacus pruinosus could be distinguished from N. nyctagineus based on only male specimens collected from the same location (Gamȏ, 1962). The Korean male specimens of N. nyctagineus undoubtedly match well with the description of N. pruinosus with the following common features: (1) the surface of the carapace is covered with numerous granules; (2) the pereonites and pleonites have similar patterns of the teeth on dorsal surface; (3) the proportion and armature of the dactylus of pereopod 2 is similar (dactylus almost 1.6 times as long as propodus, with numerus hairs, 1 lateral, 2 medial, and 4 terminal setae on surface in both sex); (4) the number and pattern arrangement of the setae on the surface in antenna 1, pereopod 1, and uropod are the all most same. Therefore, we concluded that N. pruinosus should be a synonym of N. nyctagineus that is known in Japan, Madagascar, and Malaysia (Petrescu, 2003). There are a few differences in sexual dimorphism between the females and males of N. nyctagineus: (1) the shape of the carapace is triangular in dorsal view in female, while it is rectangular in male; (2) the number of teeth on the surface of pleonite 1-2 is different (2 pairs of teeth in female vs. 1 pair of teeth in male). Moreover, uropodal peduncle and endopod of females are shorter than those in males. It is possible that Gamȏ might have overlooked the sexual dimorphism that is sometimes present in Nannastacus species, such as N. antipai, N. mitreai, and N. wisseni(Petrescu, 1995, 1997).